1. Field of the Invention
The present invention relates to an exhaust and an intake system for a variable cylinder engine.
2. Description of Background Art
A conventional exhaust system for a variable cylinder engine is known that carries out a cylinder deactivation operation without deactivating an intake valve and an exhaust valve of some cylinders. See, for example, Japanese Patent Laid-Open No. Hei 7-133716. In the conventional exhaust system for a variable cylinder engine, exhaust pipes of activation cylinders and exhaust pipes of deactivation cylinders are gathered and a main catalyst is disposed on the downstream side of the gathering section. Further, a pre-catalyst is disposed in the exhaust pipes of the activation cylinders on the upstream side of the gathering section. Exhaust gas from the activation cylinders is purified by the pre-catalysts first, and then exhaust gas including unburned components that have not been purified by the pre-catalysts is joined to fresh air from the deactivation cylinders and then purified again by the main catalyst. Consequently, a purification performance is assured and a temperature drop of the main catalyst is suppressed.
If the exhaust system for a variable cylinder engine disclosed in Japanese Patent Laid-Open No. Hei 7-133716 above is adopted, then the temperature drop of the main catalyst can be suppressed. However, CO and H2O included in exhaust gas in the activation cylinders react in the pre-catalysts to produce H2, and this H2 sometimes reacts with NO included in the exhaust gas similarly to produce NH3. Further, if this NH3 is joined and reacts with fresh air in the main catalyst, then this is oxidized back into NO. Therefore, there is a problem that NO is exhausted to the downstream side of the main catalyst. Japanese Patent Laid-Open No. Hei 7-133716 describes that, as a countermeasure against this problem, a sub-catalyst is additionally provided on the downstream side of the main catalyst. However, since an increase in the number of catalyst gives rise to an increase in the cost, a requirement to assure a purification performance without increasing the number of catalysts is desired.
In addition, a variable cylinder engine is known wherein some cylinders are deactivated only by cutting fuel supply and ignition without deactivating intake valves and exhaust valves. See, for example, Japanese Patent Laid-Open No. 2002-349304. Further, a crankcase emission controlling apparatus is known wherein blow-by gas generated in a crankcase of an engine is introduced from a throttle apparatus into an air cleaner on the upstream side to return the blow-by gas into the combustion chamber together with intake air to combust the blow-by gas thereby to prevent the blow-by gas from being emitted to the atmospheric air.
Further, if the crankcase emission controlling apparatus described above is incorporated into the variable cylinder engine disclosed in Japanese Patent Laid-Open No. 2002-349304 specified above and an intake passage of an activation cylinder group and an intake passage of a deactivation cylinder group are connected to a single air cleaner, then there is the following possibility. In particular, upon deactivation of the cylinder operation, there is the possibility that blow-by gas introduced into the air cleaner may pass through the deactivation cylinder group, in which no combustion occurs, and then be emitted into the atmospheric air while it remains unburnt.